VIENNA (Reuters) - The United Nations says momentum is building for tougher long-term action to fight global warming beyond the U.N.'s Kyoto Protocol and a climate meeting starting in Vienna on Monday will be a crucial part of the process.

Negotiators from more than 100 countries at the Aug 27-31 talks will seek common ground between industrial nations with Kyoto greenhouse gas caps until 2012 and outsiders led by the United States and China, the biggest greenhouse gas emitters.

"Momentum is very much building," for wider action, Yvo de Boer, the U.N.'s top climate change official, told Reuters of the Austrian meeting of about 1,000 senior officials, scientists and activists. "And Vienna's going to be crucial."

The Vienna talks will try to break a diplomatic logjam and enable environment ministers to agree at a meeting in Bali, Indonesia, in December to launch formal two-year negotiations to define stiffer long-term curbs on greenhouse gases.

"All countries need to take more urgent action," South Africa wrote in an advance statement for the Vienna talks. "The pace of the climate negotiations is out of step with the urgency indicated by climate science."

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Chances of a deal in Bali have risen sharply after U.N. reports this year blamed human activities, led by use of fossil fuels, for a changing climate set to bring ever more severe monsoons, heatwaves, droughts and rising seas.

Prospects for an agreement on worldwide action to fight climate change have brightened since President George W. Bush, a Kyoto opponent, agreed in June with his industrial allies on a need for "substantial cuts" in greenhouse gas emissions.

It is unclear exactly what "substantial" means for Washington. The European Union, Japan and Canada have all talked about a need to halve world emissions by 2050 to slow warming.

"BALI ROAD MAP"

De Boer, who heads the Bonn-based U.N. Climate Change Secretariat, said that many nations wanted a "Bali road map" agreed in Indonesia -- a two-year plan to work out a deal to succeed Kyoto which runs to the end of 2012.

A road map could include principles that a deal should include major emitters, that it should not undermine economic growth in developing nations and that rich nations should take the lead, delegates say.

But many countries are unlikely to show their hands yet.

"We will see an exercise where countries say 'of course ambitious targets are needed, but my neighbor needs to go first'," said Hans Verolme, director of the WWF conservation group's climate change program.

Even though there are five years left until 2012, many experts say time is already running short. Anyone planning to build a coal-fired power plant, or to invest in carbon markets, wants to know the long-term rules.

Kyoto took two years to negotiate but then seven to ratify after President George W. Bush broke with his main allies in 2001 and decided not to implement the pact. He said Kyoto was too costly and wrongly excluded 2012 goals for poor states.

Vienna will also review a new U.N. report saying that by 2030 the world will have to spend hundreds of billions of dollars annually to fight climate change. Curbing greenhouse gases alone will cost some $210 billion annually, it says.

The United Nations argues that it will be far cheaper to combat climate change now than wait for widening consequences, ranging from rising seas to droughts in Africa.